Throughout Cunard Line shorts and T shirts can be worn on board up until 6pm. After that the dress code of the evening applies for access to all public rooms and diners. Some people are conscious of maintaining standards and many will smarten up for lunch or afternoon tea however there is no hard and fast rules about it. Vest type T shirts are generally frowned upon but polo shirts and sleeved t shirts are quite acceptable for most daytime events. I expect the purist cruisers will all have their own opinion and some follow-up comments are bound to appear but when you read them all you can make up your own mind. Enjoy your cruise.